1. Field of the Invention
The invention relates to a method of determining chemical and/or physical properties of a gas atmosphere, wherein the gas atmosphere to be examined is brought into contact with a heatable surface and the resulting changes in the temperature level are converted into an electrical signal.
2. Description of the Related Art
The analysis of the chemical properties of a gas atmosphere essentially involves the question of what kind of gas is in the gas atmosphere or, for gas mixtures, whether one or several certain gases are present in the gas atmosphere. Such an analysis is made, for example, in coal mining to check for the presence of pit gases, particularly methane gas. Such analyses of the composition of gas atmospheres and the checking for the presence of certain types of gas in a gas atmosphere constitute the major field of use for such methods. The analysis of the physical properties of a gas atmosphere in the sense of the present invention relates primarily to the determination of conductivity, convection, heat tone and flow velocity of the gas atmosphere to be tested. The term gas atmosphere includes a pure gas as well as gas mixtures.
In the past, such analyses of a gas atmosphere have been effected in that a wire heated by an electrical current and having thermocatalytic properties was placed into a chamber containing the gas atmosphere to be analyzed, preferably with the gas atmosphere to be analyzed flowing through the chamber. The heating wire was connected with appropriately adapted resistors in a Wheatstone bridge configuration. With the aid of such an arrangement it is possible, for example, to measure the quantity of heat carried off by a gas flowing by and, with the appropriate calibration, information can be obtained regarding the flow velocity (see Friedrich Oehme, "Chemische Sensoren" [Chemical Sensors], 1991, FIGS. 6-12).
The determination of the presence of certain types of gas in the gas atmosphere to be examined is possible with the prior art methods only if the gas atmosphere is to be examined for the presence of reactive gases, for example the presence of methane gas or the like. By setting a certain thermocatalytic temperature at the heating wire, a reactive gas will react with the remaining gases contained in the gas atmosphere; for example, the reaction of methane with the oxygen from the air results in an increase in temperature which then "detunes" the Wheatstone bridge so that a signal can be initiated there.
In addition to the above mentioned, very limited possible uses for the prior art methods, another drawback is the use of the Wheatstone bridge circuit as a measuring system. Such a bridge circuit is in itself temperature dependent to a considerable degree since the resistors connected to the heating wire by themselves are also subject to temperature influences so that the bridge circuit is detuned then as well and error measurements result if, due to extraneous temperature influences, the resistors inserted in the circuit are subjected to temperature fluctuations and thus change the zero balance of the circuit. Another drawback of the prior art manner of proceeding is that a considerable amount of current is required to constantly heat the heating wire connected with the gas atmosphere so that mobile measuring instruments require large-capacity batteries which nevertheless permit only limited use times before they must be recharged again. Another drawback of the prior art method is that the system operates with relatively slow response times. Since overheating of the heating wire may also occur which changes the behavior of the wire, the instrument must be recalibrated after every response. Such a measuring device is unstable and not suited for permanent use, for example for monitoring purposes.
It is now an object of the invention to provide a method with which practically all types of gas can be detected in a gas atmosphere and changes in the composition of the gas atmosphere can be analyzed, with current consumption as well as response time being reduced.